Conventional electric probe units for electrical inspections, such as of a printed circuit board's conductor pattern or electronic device, employ microcontactor probes constituted with a plunger as a conductive needle member, a holder for holding the plunger to be axially movable to go in and out, and a coil spring for resiliently biasing the plunger in a direction for a projecting end thereof to project from a front end (as an axial end) of the holder, thereby allowing for the projecting end of the plunger to resiliently contact a contact target of an object to be inspected.
As such an example, there has been known a microcontactor probe 100 shown in FIG. 13, as it is disclosed in Japanese Patent Application Laid-Open Publication No. 10-239349, in which a plunger assembly 5 that has a pair of plungers 3 and 4 joined to both ends of a coil spring 2 is accommodated to be installed in support holes 8 and 9 formed in insulating holders 6 and 7, allowing for reciprocal movement of the paired plungers 3 and 4, while the installed plungers 3 and 4 are urged to project outward and prevented from falling.
In the installed condition, the plungers 3 and 4 accommodated in the support holes 8 and 9 are prevented from falling, with their stepped parts engaged either with the lower holder 6 and the other with the upper holder 7. The coil spring 2 has a tight wound spiral portion 2a in an intermediate part thereof. The intermediate tight wound spiral portion 2a has a length for stem parts 3a and 4a of the plungers 3 and 4 to contact thereto in a waiting state (the state in which the plunger 3 is not brought into contact on an inspection target).
The above-noted probe 100 is used for inspection, in an electrically connected condition in which a projecting end of the upper plunger 4 resiliently contacts a lead conductor 11 that is fixed to a wiring plate 10 laminated on the holder 7, by bringing a projected end of the lower plunger 3 into resilient contact on the inspection target 12 of the object of inspection. In the inspection, a conduction path L of electronic signal develops, as shown by solid line arrows in FIG. 13, extending from the inspection target 12 to the lower plunger 3, for conduction from the stem part 3a to the step part 4a of the upper plunger 4 via the intermediate tight wound spiral portion 2a, and passing the upper plunger 4, leading to the lead conductor 11, whereby the inspection target 12 as a printed circuit formed on the substrate 13 can be inspected for short-circuiting or disconnection.
In this condition, the conduction path L that has to pass the single coil spring 2 includes the intermediate tight wound spiral portion 2a, where significant fractions of electric signals are linearly conducted in an axial direction of the coil spring 2, thus preventing a high-frequency signal from flowing a coarse wound spiral portion of the coil spring 2 which would have an additional inductance and an additional resistance.
In the microcontactor probe 100, however, the conduction path L includes, as shown in FIG. 13, lower and upper sliding portions A and B for conduction between the intermediate tight wound spiral portion 2a and the stem parts 3a and 4a of the lower and upper plungers 3 and 4, respectively, which two sliding portions A and B for conduction cause dispersion of resistance, resulting in a reduced accuracy of inspection, as a problem.
Further, the microcontactor probe 100 goes unstable if the contact pressure to be exerted by the plunger 3 on the inspection target 12 is not great to some extent, as it gives rise to a high contact resistance due to such a cause as a failure to break an oxidized skin of the inspection target 12 for example. There is thus required a great contact pressure, which however is concurrently exerted via the plunger 4 also on the lead conductor 11, which means that in application to a mass production the lead conductor 11 is subjected to a great contact pressure imposed thereon every test (that amounts to hundreds of thousands to millions times), resulting in a reduced service life at the substrate 13 (or the wiring plate 10) due to an early rupture or the like, as another problem.